There is extensive interest in expanding the range of useful products derived from the processing of trees or other cellulosic-containing materials to improve overall yields from raw material fed into lumber and pulp processing. There are many potential products that might utilize the bark, needles, sawdust, and other waste byproducts, by incorporating them into a synthetic wallboard or insulating material. The key ingredient of most such suggested products is an adhesive that can bind together the comminuted byproducts into an integral unit having cohesive and tensile strength. Such an adhesive, because of the relatively low value of the waste material utilized and the low market price which these new products would be likely to command, must be inexpensive and derived from readily available raw materials.
The commonly used adhesives in the forest products industry are those derived from the acid or base catalyzed condensation reaction of phenol with an aldehyde, usually formaldehyde or furfural. The phenolic-formaldehyde resin is water-resistant and suitable for use in exterior plywood subject to exposure to weather.
The phenol-formaldehyde resins are not usually suitable for the production of low-value products because of their high cost, with the phenol ingredient of the resin being the expensive raw material. As a derivative of petroleum, phenol's price has risen steadily. Also, its availability as an adhesive ingredient is likely to become a problem as supplies of petroleum feed stocks decline in the future.
Spent sulfite cooking liquor, a waste material derived from the processing of cellulosic containing material into pulp, has received extensive attention as an ingredient in the search for a cheaper adhesive. Waste sulfite liquor is usually generated as a byproduct from the digestion of wood chips in an aqueous solution of a sodium, calcium, magnesium or ammonium bisulfite in the presence of an excess of sulphur dioxide. In the digestion process the cellulosic fibers are released from the matrix of lignin in which they are bound in nature. The SO.sub.2 or HSO.sub.3.sup.- ion solublizes the lignin as a salt of lignosulfonic acid, ammonium lignosulfonate, for example, if the base chemical used in cooling is ammonia. After digestion, the fibers are recovered leaving behind the spent sulfite cooking liquor. Spent sulfite liquor is a solution of 10-15 percent solids comprising the lignosulfonate as the principal constituent, simple sugars such as pentoses and hexoses as well as polysaccharides, some resin and unconsumed cooking chemicals.
Concentrated waste sulfite liquor has long been known to exhibit certain adhesive properties, perhaps partially resulting from the "sticky" gums and sugar components which may constitute up to 30 percent of the liquor solids. The use of sulfite waste liquors as adhesives has not generally been successful, however, because of the relatively low quality of bonding provided. Another major disadvantage of waste sulfite liquor-derived adhesives is their high water solubility after curing.
Goss, in U.S. Pat. Nos. 3,144,873 and 3,079,353, describes a process in which dilute sulfite liquor is heated to 180.degree. C. to cause condensation of a portion of the sulfite liquor sugars to ammonia insoluble forms. This process improves the yield of usable sulfonate resin when the dried sulfite liquor solids are treated with anhydrous liquid ammonia to extract the remaining ammonia soluble sugars and other carbohydrate factions. The resulting desugared lignosulfonate is a water-soluble thermosetting resin which can be used to strengthen soft composition board without further treatment.
Another common approach in the search for a cheaper adhesive has been to utilize lignosulfonates as a substitute for a substantial portion of the expensive phenol component in phenol-formaldehyde resins. This substitution is based upon the common chemical properties that lignin and phenol possess. While the structure of lignin is not known with certainty, it may generally be described as a network polymer comprised of building blocks such as trans-coniferyl alcohol with free phenol-hydroxyl-groups that potentially promote attachment of formaldehyde on the aromatic ring. While phenol has the "2," "4," and "6" positions on the aromatic ring activated, however, the coniferyl component of lignin has only the "2" position free for reaction with formaldehyde. Therefore, a mixture of a lignosulfonate derived from waste sulfite liquor, phenol and an aldehyde will not incorporate a substantial portion of the lignosulfonate into a resulting polymer product because the phenol and aldehyde react rapidly while the lignosulfonate reacts very slowly. The finished product will most likely be a diluted phenol-aldehyde condensation product mixed with a portion of unreacted waste sulfite liquor.
An improved substitution of lignosulfonate for a portion of the phenol in phenol-aldehyde resin condensation products is achieved by pre-reacting the lignosulfonate with phenol prior to condensing with the aldehyde. This preliminary step involves heating the two components together for a period of time under relatively mild conditions and at a wide range of waste sulfite liquor to phenol concentrations. It has been suggested that the sulfonate components of the sulfite liquor condense with the phenol thereby producing a phenolated lignosulfonate resin with many more reactive sites available for the subsequent condensation with an active aldehyde.
In U.S. Pat. No. 2,385,586 to Rudy and Watzel, a dry, solid waste sulfite liquor is dissolved in phenol in a 1:1 molar ratio in the presence of large amounts of an anhydrous phosphoric acid catalyst. The reaction is carried out at 50.degree.-100.degree. C. under atmospheric pressure to yield a condensation product that is a solid resin at room temperature but readily soluble in water.
In U.S. Pat. No. 3,658,638 to Ludwig and Stout, an alkali metal lignosulfonate liquor of 35-90 weight percent solids in aqueous alkaline medium is mixed with phenol, with the lignosulfonate constituting 10-100 percent of the weight of the phenol charged. The mix is heated to the 90.degree.-125.degree. C. range under atmospheric pressure for 1-4 hours resulting in a phenolated lignosulfonate that is water soluble and reacts with formaldehyde to yield an exterior grade adhesive. The process requires that the lignosulfonate be an alkali metal salt and that the reaction be carried out under alkaline conditions. A reaction temperature of 180.degree. C. or somewhat higher may be used for the more dilute solutions of waste sulfite liquor in the 35-65 percent solids range to effect reaction to the extent desired within 2-3 hours.
In U.S. Pat. Nos. 2,772,139, and 2,794,790 to Marshall et al, a waste sulfite liquor solution of 10-50 percent solids that has been decationized is reacted with phenol, in an amount up to 200 percent in excess of that which will combine, at a temperature of 100.degree.-150.degree. C. for up to 12 hours yielding a phenolated lignosulfonate that on condensation with an aldehyde results in a water soluble thermosetting compound which on further heating becomes insoluble in water. In U.S. Pat. No. 2,772,139 it is noted that temperatures in excess of 150.degree. C. cause the reaction to proceed very rapidly, making control difficult.
In U.S. Pat. No. 3,227,667 issued to Moffitt, an acid-acetone extracted solution of lignin is reacted with a portion of acidified phenol by heating to 60.degree.-110.degree. C. The resulting water soluble condensate is then condensed with an alkalized aldehyde to give a final condensate that is soluble in the alkalized mixture but insoluble in water. It is noted that at no time is the lignin mixture exposed to a temperature above 140.degree. C.